Synthesis and Characterization of Titania Nanotubes for Photocatalytic Water-Splitting and Carbon Dioxide Methanation.

Abstract

As carbon dioxide emissions from coal-based electricity generation increase, new technologies are required to convert carbon dioxide to useable fuels, such as methane and hydrogen. The Sabatier reaction would be a useful method of reducing carbon dioxide emissions; however, the generation of the hydrogen needed for this reaction involves the use of fossil fuels. Therefore, a simultaneous method of generating hydrogen from non-fossil fuel sources and subsequent reduction of carbon dioxide to useable fuels, such as hydrogen and methane, is greatly desired. Metal-supported titania nanotubes are of particular interest due to their photocatalytic properties and high surface area for reactant adsorption. In particular, ruthenium-doped titania nanotubes are of interest because ruthenium is a highly active carbon dioxide methanation catalyst and titania offers a wide band gap. In this study, the properties and photocatalytic activity of titania nanotubes are studied as a function of synthesis conditions. Using the anodic oxidation method, titania nanotube dimensions are tailored based on synthesis conditions. In addition, correlations between the synthesis conditions and oxygen content, conductivity, and photocatalytic activity of titania nanotubes are established. Titania nanotube formed in low fluoride-containing electrolytes exhibited high oxygen content, reduced conductivity, and superior photocatalytic activity. Ruthenium-doped titania nanotubes are evaluated for use as a thermal catalyst for the Sabatier reaction and are compared ruthenium-supported alumina catalyst, the industry standard. A mechanism for simultaneous water-splitting and carbon dioxide methanation over Ru-doped titania nanotubes under UV light illumination is hypothesized based on Diffuse Reflectance Infrared Fourier Transform Spectrometry. Methane forms over Ru-doped titania nanotubes at 65°C under UV light illumination, which proves that water splitting and carbon dioxide methanation reaction can occur simultaneously over this photocatalyst. Finally, the water-splitting reaction is evaluated over Ru-doped anatase titania, Ru-doped amorphous titania nanotubes, un-doped anatase titania nanotubes, Ru-impregnated Degussa P25, and un-doped Degussa P25 photocatalysts. The physical addition of activated carbon to a flow reactor resulted 44% hydrogen generation from photocatalytic water vapor splitting over 7 cm2 of UV-illuminated Ru-doped titania nanotubes. This is a record photocatalytic conversion of water vapor to hydrogen in a non-microreactor, titania-based system.